The Princeton Neuroscience Institute (PNI) proposes to purchase a Zeiss MultiSEM, the world's fastest scanning electron microscope. Two-photon calcium imaging of neurons in behaving animals is important for many projects at Princeton, including several that are part of the U.S. BRAIN Initiative. The technique is being applied to large neural populations distributed across the entire nervous systems of small animals (worm, ?y, or larval zebra?sh), as well as to particular regions of the mouse brain (neocortex, medial temporal lobe, cerebellum, and basal ganglia). These large-scale observations of neural activity are conducted while the animal is performing complex behaviors such as navigation, decisionmaking, and courtship. The MultiSEM would add large-scale anatomical information with unprecedented resolution and completeness. The extremely high throughput of the MultiSEM would make it routine to perform serial section EM imaging of the entire brain for small animals, or of large brain regions for the mouse. It would be possible to directly relate anatomy and physiology by applying two-photon calcium imaging and the MultiSEM to exactly the same neurons. In the past, this combined approach has been hindered by the fact that 3D EM imaging generates a huge amount of data and has taken months or years for even a small volume. The bottleneck has been relieved by Zeiss' introduction of the MultiSEM, which scans with 61 beams in parallel. This is a radical increase relative to a conventional SEM, which scans with a single beam. The MultiSEM is fast enough to produce a petabyte in a few months, enough for a millimeter-scale brain volume. PNI is one of the few places in the world that is ready to handle petascale datasets from the MultiSEM, as PNI has recently received funding from IARPA to scale up EM image analysis techniques, including deep learning and crowdsourcing. The instrument will be housed in the Bezos Center for Neural Circuit Dynamics, in the basement of the new PNI building. The Bezos Center already provides two-photon microscopy technologies to laboratories at PNI, and would become an even more important resource with the MultiSEM.